Adobe Creative Suite 5 and Adobe Creative Suite 4 are independent software versions and can co-exist on the same computer. If you have purchased an upgrade to Creative Suite 5, you do not need to remove Creative Suite 4 from your computer. You can decide whether to remove Creative Suite 4 based on your available hard disk space and whether you need to work with Creative Suite 4 occasionally.
You cannot have Creative Suite 5 and Creative Suite 4 applications (for example, Photoshop CS5 and Photoshop CS4) open at the same time. However, when you close one version you can start the other version.